The present disclosure relates generally to automobile steering systems and, more particularly, to a method for detecting a loss of motor control in a power steering such as a four-wheel steering system.
A four-wheel steering system simultaneously steers the front and rear wheels of an automobile. More specifically, an electrically driven, rack and pinion rear wheel steer system produces a desired rear wheel steering angle to improve directional stability at high speeds and steering maneuverability at low speeds. Four-wheel steering methods include a xe2x80x9ccommon phasexe2x80x9d method and a xe2x80x9creverse phasexe2x80x9d steering method. The common phase steering method reduces vehicle yaw (an angular speed about a vertical line passing through a center of gravity of a vehicle) by steering the front and rear wheels in the same direction. In contrast, the reverse phase steering method achieves good steering at low speeds by steering the front and rear wheels in opposite directions, thereby reducing the turning radius of the vehicle.
When a malfunction of a four-wheel or rear-wheel steering system occurs during the operation of an automobile, a xe2x80x9creturn to centerxe2x80x9d mechanism (such as a spring) typically assists the rear wheels in returning to a neutral position. However, such a spring force by itself would tend to create a return to center steer velocity higher than desired, thus causing an undesirable disturbance to an operator of the automobile.
The above discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by a method for detecting a loss of control an electric power steering system. In an exemplary embodiment, the method includes determining a duty cycle of a steering command signal generated by a controller. The steering command signal commands a steering mechanism to be turned in either a first or a second direction, the second direction being opposite to the first direction. A steering velocity of the steering mechanism is determined, the steering velocity being characterized by a steering velocity magnitude and a steering velocity direction whenever the steering velocity is greater than zero. The steering velocity direction corresponds to either the first or the second direction. The duty cycle is then compared to a first selected value, and the steering velocity magnitude is compared to a second selected value. If the duty cycle exceeds the first selected value, the steering velocity magnitude exceeds the second selected value, and the steering velocity direction is opposite to the direction commanded by the steering command signal, then a fault signal is generated.
In a preferred embodiment, the fault signal causes a motor shorting relay to be de-energized, thereby causing the open motor phase windings to be short-circuited. The first selected value is preferably about 80% and the second selected value is preferably about 1 degree per second.